Receptors for the psychotomimetic dimethoxyphenylisopropylamine (DOI) were identified in discrete brain areas and shown to cross- displace with LSD. These receptors were shown to be of the 5HT2 type. Chronic DOI administration to rats resulted in a down regulation of cortical 5HT2 receptors. New very sensitive quantitative radioimmunocytochemical methods were developed. These methods allowed for the localization and quantification of many antigens in individual rat brain nuclei in tissue sections, by autoradiography and quantitative microndensitometry and have a 100- fold higher sensitivity than classical RIA. Quantitative autoradiography was utilized for the study of amine and neuropeptide receptors in human lymphocytes and platelets, with a ten-fold increase in sensitivity over previous assays. We have profound changes in paraventricular nucleus angointensin II receptors in conditions of altered CRF secretion in rats, suggesting a role for central and peripheral angiotensin in CRF release and in stress. We have described and characterized angiotensin II and atrial natriuretic peptide receptors in sympathetic ganglia of the rat and found alterations in these receptors in conditions of altered sympathetic activity, suggesting a role for these peripheral/local hormones in the regulation of autonomic function. We applied quantitative autoradiographic techniques to study the classification of beta-adrenocreptors in discrete brain areas and parts of the conduction system of the heart.